Lettuce, Lactuca sativa L., is a commercially important fresh leaf crop belonging to the aster family (Asteraceae) which includes such other important crops as sunflowers and artichokes. Lettuce is widely grown throughout the temperate and subtropical regions of the world, and is used predominantly as a fresh green in the human diet.
Lettuce originated from the ancestral wild species L. sativa. Today there are over one hundred cultivars which are divided in commerce into four large groups based on gross morphological characteristics of the gross leaf morphology and leaf arrangement: the Romaine (or Cos) Group, the Iceberg (or Crisp Head) Group, the Butterhead Group and the Loose Leaf Group. These basic lettuce types frequently form the basis for grouping lettuces as is commonly seen in supermarkets, grocery and produce stores. Each of these basic groups is comprised of numerous cultivars, each characterized by its own particular morphology, disease resistance, and cultural adaptations.
The present invention is directed to a novel cultivar within the Iceberg or Crisp Head lettuce group, by far the most popular group of lettuces sold. Iceberg Group lettuces are characterized basically by their relatively large, firm well formed heads. The leaves are spirally arranged on a stem with greatly foreshortened internodes, are tightly clasping upon one another and have a large water content, hence the name "Crisp Head". Within the Iceberg Groups there are several clusters or assemblages of related cultivars. One of these clusters is the Vanguard-type, named after the Vanguard cultivar which has served as one parent of the other Vanguard-type cultivars. Vanguard-type lettuce cultivars share the common characteristics of being adapted to the soils and climates of the desert growing region of western North America and possess in common certain morphological characteristics including:
1. Dark, dull-green outer leaves; PA1 2. Coloration of leaves extending well down the leaf base towards the core; PA1 3. Margins of the leaves being scalloped or undulate; PA1 4. A softer leaf texture as compared to cultivars in the Great Lakes group; PA1 5. Heads with creamy colored interiors.
Problems with existing cultivars adapted to western conditions include a general lack of significant resistance to corky root rot. This disease is caused by a pathogenic soil bacterium of the genus Rhizomonas and accounts for significant lettuce crop loss in the western United States, particularly in the Salinas and Santa Maria valleys. The bacterium attacks and destroys much of the root system, greatly reducing the ability of the lettuce plant to take up water and nutrients. Loss of the root system results in uneven stunted plants that are chlorotic and too small to harvest.
Unlike other leaf crops which are cooked and thus may be canned or otherwise processed, lettuce is almost universally sold as a fresh vegetable. Thus, its market ready appearance and perishability are key factors governing salability. Existing cultivars are often inferior because they produce heads which are undesirably small or which have loose or otherwise misshapen leaves. Leaf color of both external and internal head leaves is important for the reason that consumers desire richly colored greens for their salads and will pass over lettuce heads which are pale, clear or milky colored.
Existing Vanguard-type cultivars fail to exhibit all of the desired characteristics necessary for optimal production during the summer growing season. Vanguard is noted for its excellent coloring, particularly on leaves inside the head, its flat ribs and high quality, but is susceptible to a host of diseases including corky root rot, downy mildew, and tipburn, and is adapted for early spring harvest in the climate and soils of California's Imperial Valley and adjacent desert regions. Sea Green is most notable for its rich green color and its moderate resistance to big vein, but it is susceptible to corky root rot and tipburn, and is harvestable only in the spring. Salinas has been a popular cultivar adapted to the soils and climate of coastal California and is characterized by a resistance to bolting and tip burn, and by a dull green outer leaves with cream-colored interior leaves, and a relatively small head size of less than 13.5 cm. Salinas is not resistant to corky root rot. Montello is somewhat resistant to corky root rot, but is adapted to muck soils in the midwest and northeastern United States. It cannot be grown commercially in the coastal valleys of California.
Although Salinas and cultivars derived from it are adapted to the climates and soils of coastal California, there exists a need for an improved coastal cultivar of Iceberg-Group lettuce that has heads which are large in size and have a rich green color; which have more uniformly shaped, tightly clasping leaves which are adapted to summer growth and harvest; and which are resistant to corky root rot.